Energy storage plant and process

ABSTRACT

An energy storage plant includes a casing for the storage of a working fluid other than atmospheric air, in a gaseous phase and in equilibrium of pressure with the atmosphere; a tank for the storage of said working fluid in a liquid or supercritical phase with a temperature close to the critical temperature; wherein said critical temperature is close to the ambient temperature. The plant is configured to carry out a closed thermodynamic cyclic transformation, first in one direction in a charge configuration and then in the opposite direction in a discharge configuration, between said casing and said tank; wherein in the charge configuration the plant stores heat and pressure and in the discharge configuration generates energy.

FIELD OF THE INVENTION

Object of the present invention is a plant and a process for the storageof electrical energy. More precisely, object of the present invention isa system able to absorb/use electrical energy from a network or a systemwhen an excess of availability and/or scarcity of consumption ismanifested, able to maintain in the time the stored energy and able totransform it back into electrical energy and to put it back in thenetwork when said electrical energy is demanded. In details, thisinvention is related to a system of storage of electrical energy in theform of potential energy (pressure) and thermal/thermodynamic energy.The present invention is part of medium and large scale energy storagesystems, for both terrestrial and marine applications, typically withpowers from hundreds of kW up to tens of MW (e.g. 20-25 MW), but alsohundreds of MW, and with storage capacity from a few hundred kWh, up tohundreds of MWh and even up to several GWh. The present invention canalso be placed in the field of small-scale energy storage systems, fordomestic and commercial applications, both terrestrial and marine,typically with powers from a few kW up to a few hundred kW and withstorage capacity from a few kWh, up to hundreds of kWh.

DEFINITIONS

The following definitions will be used in the present description and inthe accompanying claims.

-   -   Thermodynamic cycle (CT): thermodynamic transformation from a        point X to a point Y, where X coincides with Y; the CT, unlike        the TTC (Thermodynamic Cyclic Transformation) below, has no mass        accumulations (significant for energy purposes) within the        cycle, while the TTC typically works between two storages of        working fluid, one initial and the other final;    -   Thermodynamic Cyclic Transformation (TTC): thermodynamic        transformation from a point X to a point Y and from a point Y to        a point X, without necessarily passing through the same        intermediate points;    -   Closed CT and/or TTC: with no mass exchange (significant for        energy purposes) with the atmosphere;    -   Open CT and/or TTC: with mass exchange (significant for energy        purposes) with the atmosphere.

BACKGROUND OF THE INVENTION

Recently, due to the ever-increasing diffusion of systems for theproduction of energy from renewable sources and in particular from windand photovoltaic sources, which are characterized by productionvariability and unpredictability, electric energy storage systems arebecoming increasingly important.

The electrical energy storage systems may perform various fundamentalfunctions for the networks, both isolated and interconnected, includingthe adjustment of frequency/supply of dynamic inertia, the supply of“flexible ramping” systems, i.e. allowing the start of emergencyproduction systems, “energy shifting” from hours of greater productionand less demand to hours which, on the other hand, present greaterdemand and/or lack of production, seasonal compensations, etc.

In addition to systems that operate according to electrochemicalprinciples (Batteries) that typically have high costs and limited usefullife, mechanical (flywheel) suitable only for small amounts of storedenergy, the systems currently in use or under development or otherwiseknown include the following.

The systems mainly in use are the hydroelectric pumping storage systems(PUMPED HYDRO STORAGE—PHS), which currently cover more than 90% of theglobally installed storage capacity. These systems are suitable for bothlong and short term storage, are quite competitive in terms of costs,but have the disadvantage of being able to be built only in places thathave particular geo-morphological conditions. Said PHS system can becounted among the energy storage systems in potential form and inparticular gravitational. Also the system disclosed in document GB2518125 A is in the family of gravitational systems.

A second system in use is the so-called CAES system (Compressed AirEnergy Storage), which consists of an Open TTC that accumulates throughtransformation into potential energy (pressure) and (possibly) thermalenergy. This CAES system is known both in the basic (non-adiabatic)configuration and in the more advanced AA-CAES (Advanced Adiabatic CAES;see U.S. Pat. No. 4,147,205—Compressed Air Storage Installation)configuration. These systems are suitable for both long and short termstorage, are quite competitive in terms of costs, are less efficientthan PHS systems in terms of ‘Round Trip Efficiency’, and they also havethe disadvantage of being able to be built only in places withparticular geo-morphological conditions.

CAES systems also have an additional disadvantage in that the pressureof the tank/cave varies with the charge level of the same. This affectsboth the efficiency of the TTC and the efficiency of the turbomachinesthat perform it.

Systems are also known to remedy the absence of underground caves forCAES systems. In particular, solutions are known that seek to make iteconomically viable to store energy in over-ground tanks, without theneed for underground caves. An example is in US2011/0204064 A1 ofLIGHTSAIL where tanks of special construction are proposed in order totry to contain costs of over-ground storage tanks that vice versa wouldmake the costs of said CAES over ground systems unprofitable. Thesesolutions also belong to the systems that work according to an Open TTC.

Systems that combine the two previous systems are also known (see U.S.Pat. No. 7,663,255 B2), in which the combination of CAES and PHS alsoallows the CAES system to operate at constant compression pressure.These systems too work according to an Open TTC.

The document ‘Novel concept of compressed air energy storage andthermos-electric energy storage’—THESE N.5525 (2012)—Ecole PolytechniqueFederale de Lousanne, discloses all types of CAES energy storagesystems. Among others, the CAES systems diabatic, adiabatic, isothermaland combined with PHS to allow a constant compression pressure, aredisclosed, this system is called Constant Pressure-CAES combined withPHS. These too are systems working according to an Open TTC.

The same document also discloses the so-called TEES (Thermo ElectricEnergy Storage) proposed by ABB Corporate Research Center (see also EP2532843 A1 and EP 2698506 A1). This is one of the systems that workaccording to a Closed CT, and can be counted among the PHES systems.PHES systems (pumped heat electrical storage) are systems for storingelectrical/mechanical energy by converting it into thermal energy using,for example, Rankine, Brayton or Kalina CT. In addition to the systemsdescribed above that use trans-critical and super-critical CO₂ cycles orother fluid cycles and therefore reversible trans-super-critical Rankinecycles, PHES systems with Brayton cycle are known, typically using Argonbut also air (see Isoentropic EP 2220343 B1 and US 2010/0257862 A1 andLaughlin US 2016/0298455 A1. This is one of the systems that workaccording to a Closed CT, and can be counted among the PHES systems).

Another system that can be counted among the PHES/TEES systems is theSiemens—Gamesa system (see US 2014/0223910 A1 and U.S. Pat. No.8,991,183 B2 and U.S. Pat. No. 8,966,902 B2) which combines twodifferent cycles for the charging and discharging phase, and inparticular provides for a Brayton cycle or simple dissipation withelectrical resistances for the charging phase of the high-temperatureheat storage tank and a steam Rankine cycle for the discharge/productionphase of electrical energy. This type of solution is one of the PHESsystems. It is carried out by means of several Open and/or Closed CT.

It should be noted that all PHES systems, also called TEES, are based ona ‘closed’ and reversible thermodynamic cycle principle. Depending onthe different proposed solutions, they can be ‘closed’ Rankine orBrayton cycles, but in any case the working fluid of the motor/heatpump, which is almost reversible, performs transformations according toa ‘closed’ thermodynamic cycle in which there are no intermediateaccumulations sized according to the required storage capacity.

All CAES systems, of all types, are instead systems that carry outtransformations, first in one direction and then in the other accordingto an ‘open’ thermodynamic cycle, that is, taking and returning air tothe atmosphere.

Another known method of energy storage is the so-called LAES system(Liquid Air Energy Storage, see US2009/0282840 A1). The LAES methodinvolves transformations according to an ‘open’ thermodynamictransformation, i.e. taking and returning air to the atmosphere.Moreover, this system works at cryogenic temperatures, close to −200°C., with high technical difficulties. This too belongs to the systemsthat work according to an Open TTC.

In ‘Analysis of the exergy efficiency of a super-critical compressedcarbon dioxide energy-storage system based on the orthogonal method’ byQing He, Yinping Hao, Hui Liu, Wenyi Liu, the use of CO₂ as workingfluid for energy storage systems was also proposed. The proposed system(called SC-CCES (Super Critical—Compressed Carbon dioxide EnergyStorage), uses as specified “two saline aquifers as storage reservoirs”.In this SC-CCES system, CO₂ from the compressor delivery is sentdirectly to the reservoir without the interposition of any heatexchanger and/or thermal energy storage system. Moreover, during thedischarge cycle, the CO₂ discharged from the turbine heats up through arecuperator the same CO₂ entering the turbine. This solution belongs tosystems that work according to a Closed TTC, i.e. between two closedtanks.

Also document ‘Green Energy Storage: “The Potential Use of compressedLiquid CO2 and Large Sub-Terrain Cavities to Help Maintain a ConstantElectricity Supply”—Dalgaard J Z, talks (both in the title and abstract,and in the body of the document) about the use of CO₂ in undergroundcavities.

SUMMARY

The applicant pointed out that the current electrical energy storagesystems do not have characteristics that allow them to be usedeconomically in different situations. In particular, in some cases (e.g.PHS and CAES) the systems require very particular geo-morphologicalsituations that are difficult to find. In some cases (e.g. PHS) theimplementation of such systems requires the manufacture of reservoirswith heavy environmental impact.

In other cases (AA-CAES) the realization of thermal energy storagesystems presents problems that are difficult to solve at low cost and,moreover, there is still the need to identify suitable undergroundcaves. The above also leads to difficulties in achieving satisfactoryRound Trip Efficiency (RTE). In any case, the problem of working withvariable pressures in the storage tank remains, unless the CAES systemis combined with the PHS system, with obvious additional costcomplications and to identify the correct geological conditions.

The applicant further observed that attempts to build surface CAESsystems have come up against the practical impossibility of buildingpressurized air storage tanks at a competitive cost to enable thesystems themselves to be built.

The applicant further observed that the attempts to build LAES systemshave not at the moment allowed to develop economically viable systemsalso because of the problems inherent in working in cryogenicconditions. The problems of storing cryogenic energy in double layertanks with vacuum between layers, and other expensive devices, makes thetechnology difficult to optimize in terms of costs.

The applicant further observed that the attempts to build PHES systemswith almost reversible Rankine cycles present considerable difficultiesin achieving satisfactory Round Trip Efficiency (RTE) (i.e. above 60%)and at the same time with reasonable costs, the RTE being linked totemperature differences in the equipment.

Similarly, PHES systems based on the Brayton cycle have to contend withthe fact that these systems use a compressor and a turbine for eachcycle, both for charging and for discharging. This entails higherinvestment costs, but also greater irreversibility that can becompensated for obtaining high TENs only by maintaining very hightemperature differences between hot and cold storage.

In this context, the Applicant has set itself the objective of designingand implementing an energy storage process and plant, i.e. an energystorage system, that is:

-   -   capable of being done in different geo-morphological situations,        which do not require particular geographical or territorial        conditions to be realized and that can eventually in certain        sizes also be used for marine/off-shore applications;    -   able to obtain high RTE and in any case higher than 70% and up        to 75% and up to even 80% and more;    -   capable of working with adjustable storage tank pressures,        through various systems described below;    -   simple and economical, preferably with the target of having a        construction cost of less than 100 USD/kWh and, in particular,        that allows storage under pressure and with high energy density        (in terms of m³ _(storage)/kWh_(stored));    -   able to increase its RTE by using the variations of Ambient        Temperature;    -   safe and environmentally friendly, e.g. that it does not use        particularly hazardous fluids;    -   modular;    -   compact;    -   lasting or having an increased useful life of 30 years;    -   flexible and able to get into operation quickly;    -   easily and economically maintainable;    -   corrosion resistant (especially for marine applications);    -   having low levels of vibrations and noise.

The applicant has found that the above objectives and others can beachieved through an Energy Storage system operating by means ofthermodynamic cyclic transformations (TTC), first in one direction andthen in the opposite direction, between two accumulations of a workingfluid in two distinct tanks, one of which (the one with the lowestpressure) is atmospheric, but which is not atmospheric air but anothergas in pressure equilibrium with the atmosphere. This system is alsocharacterized by the fact that it stores energy transforming the workingfluid from an initial gaseous/steam state to a final liquid orsupercritical state with a temperature close to critical temperature(for example less than 1.2 times the critical temperature in Kelvin,preferably between 0.5 and 1.2 times). It is also characterized by thefact that this critical temperature is preferably not far from ambienttemperature, preferably close to ambient temperature (preferably between0° C. and 200° C., more preferably between 0° and 100° C.).

The working fluid is preferably carbon dioxide (CO₂), but in order toimprove the performance of the system, also in relation to theparticular environmental conditions in which it operates, a mixture ofCO₂ and other substances could be used in order to correct the criticaltemperature T_(c) of the fluid. Other fluids, such as SF₆, N₂O, etc. canbe used, always pure or mixed with others.

In the system proposed in this invention, a storage of heat recoveredfrom the delivery of a compressor is present. Both high and low pressuretanks work at constant pressures or in any case adjusted within certainwell-defined “ranges”, both when the system operates in subcritical andsupercritical conditions, possibly with different control strategies.

In particular, the stated objectives and others are substantiallyachieved by a plant and a process for energy storage of the type claimedin the attached claims and/or described in the following aspects.

In an independent aspect, the present invention concerns an energystorage plant (energy storage system).

Preferably, the plant comprises:

a casing for the storage of a working fluid other than atmospheric air,in a gaseous phase and in equilibrium of pressure with the atmosphere;

a tank for the storage of said working fluid in liquid or supercriticalphase with a temperature close to the critical temperature (for exampleless than 1.2 times the critical temperature in Kelvin 0.5-1.2); whereinsaid critical temperature is between 0° C. and 200° C., more preferablybetween 0° C. and 100° C., preferably close to the ambient temperature;

wherein the plant is configured to carry out a closed cyclicthermodynamic transformation (TTC), first in one direction into a chargeconfiguration/phase and then in the opposite direction into a dischargeconfiguration/phase, between said casing and said tank; wherein in thecharge configuration the system accumulates heat and pressure and in thedischarge configuration it generates energy.

Preferably, the working fluid has the following chemical-physicalproperties: critical temperature between 0° C. and 100° C., density at25° C. between 0.5 and 10 Kg/m³, preferably between 1 and 2 Kg/m³.

Preferably, the working fluid is chosen in the group including: CO₂,SF₆, N₂O, or a mixture of the same, or even a mixture of the same withother components that act as additives, for example mainly to modify theparameters of critical Temperature of the resulting mixture in order tooptimize the performance of the system.

Preferably, the energy storage plant comprises:

-   -   a compressor and a motor mechanically connected to each other;    -   a turbine and a generator mechanically connected to each other;    -   said casing externally in contact with the atmosphere and        delimiting inside a volume configured to contain the working        fluid at atmospheric pressure or substantially atmospheric        pressure, wherein said volume is selectively in communication        with an inlet of the compressor or with an outlet of the        turbine;    -   a primary heat exchanger (or even a plurality of primary heat        exchangers that may also operate with different fluids on their        secondary side) selectively in fluid communication with an        outlet of the compressor or with an inlet of the turbine;    -   said tank in fluid communication with the primary heat exchanger        to accumulate the working fluid;    -   a secondary heat exchanger operationally active between the        primary heat exchanger and the tank or in the tank.

This plant is configured to operate in a charge or in a dischargeconfiguration.

In the charge configuration, the casing is in fluid communication withthe inlet of the compressor and the primary heat exchanger is in fluidcommunication with the outlet of the compressor, the turbine is at rest,the motor is operating and drives the compressor to compress the workingfluid coming from the casing, the primary heat exchanger works as acooler to remove heat from the compressed working fluid, cool it andstore thermal energy, the secondary heat exchanger works as a cooler toremove additional heat from the compressed working fluid and storeadditional thermal energy, the tank receives and stores the compressedand cooled working fluid, wherein the working fluid stored in the tankhas a temperature close to its own critical temperature (e.g. between0.5 and 1.2 of the critical Temperature in Kelvin). In the dischargeconfiguration, the casing is in fluid communication with the outlet ofthe turbine and the primary heat exchanger is in fluid communicationwith the inlet of the turbine, the compressor is at rest, the secondaryheat exchanger works as a heater to release heat to the working fluidcoming from the tank, the primary heat exchanger works as a heater torelease further heat to the working fluid and heat it, the turbine isrotated by the heated working fluid and drives the generator generatingenergy, the working fluid returns in the casing to atmospheric pressureor substantially atmospheric.

In an independent aspect, the present invention relates to an energystorage process, optionally implemented with the plant according to theprevious aspect or according to at least one of the following aspects.

Preferably, the process comprises: carrying out a closed thermodynamiccyclic transformation (TTC), first in one direction in a chargeconfiguration/phase and then in an opposite direction in a dischargeconfiguration/phase, between a casing for the storage of a working fluidother than atmospheric air, in a gaseous phase and in equilibrium ofpressure with the atmosphere, and a tank for the storage of said workingfluid in a liquid or super-critical phase with a temperature close tothe critical temperature (for example between 0.5 and 1.2 of theCritical Temperature in Kelvin);

wherein said critical temperature is close to the ambient temperature,preferably between 0° C. and 100° C., but also up to 200° C.; whereinthe process accumulates heat and pressure in the charge phase andgenerates energy in the discharge phase.

Preferably, said working fluid has the following chemical-physicalproperties: critical temperature between 0° C. and 200° C., morepreferably between 0° C. and 100° C., preferably close to ambienttemperature.

Preferably, this working fluid is chosen in the group including: CO₂,SF₆, N₂O, or a mixture of the same, or even a mixture of the same withother components that act as additives, for example mainly to modify theparameters of critical Temperature of the resulting mixture in order tooptimize the performance of the system.

Preferably, the process comprises a phase of energy charge and a phaseof discharge and generation of energy.

The charge phase comprises:

-   -   compressing the working fluid, coming from said casing        externally in contact with the atmosphere and delimiting inside        a volume configured to contain said working fluid at atmospheric        pressure or substantially atmospheric, absorbing energy;    -   injecting the compressed working fluid through a primary heat        exchanger (or even a plurality of primary heat exchangers        eventually operating with different fluids on their secondary        side) and a secondary heat exchanger placed in series to bring a        temperature of the working fluid near its own critical        temperature; wherein the primary heat exchanger works as a        cooler to remove heat from the compressed working fluid, cool it        and store thermal energy, wherein the secondary heat exchanger        works as a cooler to remove further heat from the compressed        working fluid and store further thermal energy;    -   accumulating the cooled working fluid in said tank; wherein the        secondary heat exchanger and the primary heat exchanger operate        a super-critical transformation of the working fluid so that        said working fluid is accumulated in the super-critical phase in        the tank or wherein the secondary heat exchanger and the primary        heat exchanger operate a sub-critical transformation of the        working fluid so that said working fluid is accumulated in the        liquid phase in the tank (preferably also with the aim of        regulating the pressure to a relatively minimum/low value).

The phase of discharge and power generation comprises:

-   -   passing the working fluid, coming from the tank, through the        secondary heat exchanger and the primary heat exchanger; wherein        the secondary heat exchanger works as a heater to transfer heat        to the working fluid coming from the tank (preferably also with        the aim of regulating the pressure to a relatively high/maximum        value), wherein the primary heat exchanger works as a heater to        release additional heat to the working fluid and heat it up;    -   passing the heated working fluid through a turbine, wherein the        turbine is rotated by the heated working fluid and drives a        generator generating energy, wherein the working fluid expands        and cools in the turbine;    -   re-injecting the working fluid from the turbine into the casing        at atmospheric or substantially atmospheric pressure.

The applicant has verified that the process and the apparatus accordingto the invention allow to achieve the set objectives.

In particular, the applicant has verified that the invention allows thestorage of energy in places without particular geo-morphologicalcharacteristics, even for marine/off-shore applications, in a safemanner and with a low environmental impact.

The applicant has also verified that the manufacture and subsequentmaintenance of an apparatus according to the invention are relativelyinexpensive.

The applicant has also verified that the invention enables high RTE tobe achieved.

The applicant has also verified that the invention allows to operate anenergy storage with the possibility to regulate the pressure in thestorage tanks, thus allowing a better operability of the system, agreater efficiency of both the turbomachinery and the system in terms ofRTE.

Aspects of the invention are listed below.

In one aspect, the primary heat exchanger is, or is operatively coupledto, a thermal storage (Thermal Energy Storage—TES).

In one aspect, first pipelines develop between the casing and thecompressor inlet and between the casing and the turbine outlet toconnect the fluid casing with the compressor and turbine.

In one aspect, at least one valve is operationally placed on said firstpipelines to connect the fluid alternately with the compressor casing orthe turbine with the casing.

In one respect, second pipelines develop between the turbine inlet andthe primary heat exchanger and between the compressor outlet and theprimary heat exchanger to put in fluid communication said primary heatexchanger with said compressor and turbine.

In one aspect, at least one valve is operationally placed on said secondpipelines to put in fluid communication the compressor with the primaryheat exchanger or the primary heat exchanger with the turbine.

In one aspect, third pipelines develop between the primary heatexchanger and the secondary heat exchanger to put in fluid communicationsaid primary heat exchanger with said secondary heat exchanger.

In one aspect, an additional heat exchanger is operationally placedbetween the casing and the compressor and between the casing and theturbine to pre-heat the working fluid before compression in thecompressor, in the charge configuration, or to cool the working fluidcoming from the turbine, in the discharge configuration.

In one aspect, the additional heat exchanger is operationally associatedwith the first pipelines.

In one aspect, the additional heat exchanger comprises an additionalthermal energy storage device.

In one aspect, in the charge configuration, the additional heatexchanger works as a heater to pre-heat working fluid.

In one aspect, in the discharge configuration, the additional heatexchanger works as a cooler to cool the working fluid and storeadditional thermal energy that is used in the charge configuration topre-heat said working fluid.

In one aspect, a cooler is placed on a branch of the first pipelinesconnected to the outlet of the turbine.

In one aspect, a further heat exchanger operatively associated with anadditional heat source is operatively interposed between the turbine andthe primary heat exchanger and is configured to further heat the workingfluid in the discharge phase before entering the turbine.

In one aspect, in the discharge configuration, the additional heatsource provides additional heat to the working fluid.

In one aspect, in the discharge phase and generation of energy, betweenthe primary heat exchanger and the turbine, it is envisaged to furtherheat the working fluid via an additional heat source.

In one aspect, the additional heat source is: a solar source (e.g. solarfield) and/or industrial waste heat recovery (Waste Heat Recovery)and/or exhaust heat from gas turbines (GT).

In one aspect, a temperature at which the working fluid is brought inthe discharge phase and just before entering the turbine, via theadditional heat source and the further heat exchanger, is greater than atemperature of the working fluid at the end of compression during thecharge phase.

In one aspect, the temperature at which the working fluid is brought viathe additional heat source and the additional heat exchanger is greaterof about 100° C., but also 200° C. or 300° C. or 400° C. compared to thetemperature of the working fluid at the end of compression.

The Applicant has verified that the further heating of the working fluidby the additional heat source allows to considerably increase the RoundTrip Efficiency (RTE).

In one aspect, the casing is deformable.

In one aspect, the casing has the structure of a gasometer.

In one aspect, the casing is a pressure-balloon.

In one aspect, the casing is made of flexible material, preferablyplastic, e.g. PVC coated polyester fabric.

In one aspect, the motor and generator are distinct elements, whereinthe motor is preferably permanently connected to the compressor and thegenerator is preferably permanently connected to the turbine.

In one aspect, the motor and the generator are defined by a singlemotor-generator. In one aspect, the plant comprises connection devices,preferably of the clutch type, between the motor-generator and thecompressor and also between the motor-generator and the turbine toconnect mechanically and alternately the motor-generator to thecompressor or to the turbine.

In one aspect, the motor-generator, the compressor and the turbine arearranged on a same axis.

In one aspect, the compression of the working fluid in the compressor isadiabatic, inter-cooled or isothermal.

In one aspect, the working fluid expansion in the turbine is adiabatic,inter-heated or isothermal.

In one aspect, an auxiliary thermal storage (Thermal Energy Storage TES)is connected to the compressor and to the turbine.

In one aspect, the auxiliary thermal accumulator is configured torealize, in the compressor and during the charge phase, an inter-cooledcompression, with one or more inter-coolings.

In one aspect, the auxiliary thermal accumulator is configured toperform, in the turbine and during the discharge phase, an inter-heatedexpansion, with one or more inter-heatings.

In one aspect, it is envisaged to perform a plurality of inter-coolingsin the charge phase and to perform a smaller number of inter-heatingsthan the number of inter-coolings using heat (accumulated in theauxiliary thermal accumulator) of only part of the inter-coolings.

In one aspect, it is envisaged to perform a plurality of inter-coolingsin the charge phase and to perform a single inter-heating in thedischarge phase by using heat (accumulated in the auxiliary thermalaccumulator) of the last inter-cooling only.

The Applicant has verified that the combination of the further heatingof the working fluid by the additional heat source together with theinter-coolings and the above mentioned inter-heatings allows to increasethe Round Trip Efficiency (RTE) up to values greater than 100%.

In one aspect, the primary heat exchanger is or comprises a fixed ormoving bed heat regenerator

In one aspect, the fixed or moving bed heat regenerator comprises atleast one thermal mass lapped by the working fluid.

In one aspect, the fixed or moving bed heat regenerator comprises atleast one thermal mass not lapped by the working fluid, but separatedfrom it by a wall, typically made of metal, which is capable ofcontaining the pressure, and therefore the mass is at atmosphericpressure.

In one aspect, the thermal mass comprises incoherent material,optionally gravel or metal or ceramic balls.

In one aspect, the thermal mass comprises coherent material, optionallycement or ceramic or metal.

In one aspect, the primary heat exchanger comprises a primary circuitcrossed by a primary fluid or several primary circuits crossed byseveral primary fluids, optionally water, oil or salts.

In one aspect, the primary circuit comprises a heat exchange portionconfigured to exchange heat with the working fluid.

In one aspect, the primary circuit comprises at least one primarystorage chamber, preferably two storage chambers, for said primaryfluid.

In one aspect, the primary circuit comprises a hot primary storagechamber, for the hot primary fluid accumulated after removing heat fromthe working fluid in the charge configuration/phase of theapparatus/process, and a cold primary storage chamber, for the coldprimary fluid accumulated after transferring heat to the working fluidin the discharge configuration/phase of the apparatus/process.

In one aspect, the primary circuit comprises a fixed bed heatregenerator, preferably operating at atmospheric pressure, which islapped by the primary fluid.

In one aspect, the secondary heat exchanger comprises a secondarycircuit crossed by a secondary fluid, optionally air or water.

In one aspect, the secondary circuit comprises a heat exchange portionconfigured to be lapped by the working fluid.

In one respect, the secondary circuit comprises at least one secondarystorage chamber for this secondary fluid.

In one aspect, the secondary circuit comprises a hot secondary storagechamber, for the hot secondary fluid accumulated after removing heatfrom the working fluid in the charge configuration/phase of theapparatus/process, and a cold secondary storage chamber, for the coldsecondary fluid accumulated after releasing heat to the working fluid inthe discharge configuration/phase of the apparatus/process.

In one aspect, the secondary heat exchanger is located between theprimary heat exchanger and said tank.

In one aspect, the secondary heat exchanger is integrated into the tank.

In one aspect, the secondary heat exchanger is equipped with systems forregulating the flow rate and/or temperature of secondary fluid,typically water or air, capable of regulating the pressure in thestorage tanks within certain limits, when the system operates insub-critical conditions.

Temperature control can be carried out by adding heat from theatmosphere or removing heat to the atmosphere, also taking advantage ofthe normal fluctuations in ambient temperature of air and water atdifferent times of the day.

In one aspect, the secondary heat exchanger is placed in a basin full ofwater, consisting of one chamber or two chambers. In said secondary heatexchanger the working fluid is condensed during charge phase andevaporated in discharge phase by circulating water, preferably throughimmersion pumps. The two chambers of said basin may be covered oruncovered and in communication or not with the environment so that thechamber from which the water is circulated for the condensation incharge phase is always cooled by the surrounding environment, while thatfrom which the water is circulated for evaporation in discharge phase isalways heated by the surrounding environment and possibly kept warm bycovering.

In one aspect, the above can be further supported by special exchangesystems that absorb heat or release heat, in a convective and radiantway with the environment, all for improving the RTE of the system. Thisway a pressure adjustment when the system is operating undersub-critical conditions is performed.

In one aspect, the heat exchange portion of the secondary heat exchangeris housed inside the tank.

In one aspect, the secondary circuit is configured to remove heat fromthe working fluid, in the charge configuration, or to transfer heat tothe working fluid, in the discharge configuration, at a temperaturebelow 100° C., optionally between 0° C. and 50° C., optionally at atemperature close to the ambient temperature.

In one aspect, in the charge configuration/phase, since the secondaryheat exchanger works in conditions close to ambient temperature, due tothe fact that the fluid has a critical temperature close to ambienttemperature, it is possible that the phase of heat removal through thesecondary heat exchanger is assisted by a phase of direct or indirectexchange with the atmosphere.

In one aspect, in the discharge configuration/phase, since the secondaryheat exchanger works in conditions close to ambient temperature, due tothe fact that the fluid has a critical temperature close to ambienttemperature, it is possible that the phase of heat supply through thesecondary heat exchanger is assisted by a phase of direct or indirectexchange with the atmosphere.

In one aspect, the tank is spherical or substantially spherical.

In one aspect, the tank is cylindrical or substantially cylindrical.

In one aspect, an outer wall of the tank is made of metal.

In one aspect, a temperature of the working fluid accumulated in thetank is between 0° C. and 100° C.

In one aspect, a pressure the working fluid accumulated in the tank isbetween 10 bar and 150 bar, preferably between 10 bar and 150 bar,preferably between 50 and 100 bar, preferably between 65 and 85 bar.

In one aspect, a ratio between a density of the working fluid whencontained in the tank and a density of the working fluid when containedin the casing is between 200 and 500.

In one aspect, the secondary heat exchanger and the primary heatexchanger are configured to operate a super-critical transformation ofthe working fluid so that said working fluid is accumulated in the tankin super-critical phase.

In one aspect, it is provided for removing heat from the working fluidin the primary heat exchanger until it is brought, in a T-S diagram, toa temperature above the critical temperature and above the Andrews bell.

In one aspect, it is provided for removing heat from the working fluidin the secondary heat exchanger by bringing it into the super-criticalphase and making it follow the right portion of the Andrews bell.

In one aspect, the tank comprises a separation membrane configured tointernally separate the tank into a first chamber with variable volumefor the working fluid in super-critical phase and a second chamber withvariable volume in fluid communication with a compensation circuitcontaining a non-compressible fluid, optionally water.

In one aspect, the compensation circuit is configured to maintain asubstantially constant pressure in the super-critical working fluidcontained in the first variable volume chamber of the tank, or at leastto maintain the working fluid pressure always above a certain minimumvalue.

In one aspect, the compensation circuit comprises an auxiliary tank forthe non-compressible fluid, optionally at atmospheric pressure, in fluidcommunication with the second variable volume chamber.

In one aspect, the compensation circuit comprises an auxiliary turbineconnected to an auxiliary generator and configured to be rotated by theincompressible fluid coming from the second variable volume chamber inthe charge configuration/phase of the apparatus/process.

In one aspect, the expansion energy of the liquid (typically water) ofthe compensation circuit in charge phase is between 1/100 and 7/100 ofthe charging energy of the storage system through the compressor.

In one aspect, the compensation circuit comprises a pump connected to anauxiliary motor and configured to pump the non-compressible fluid fromthe auxiliary tank into the second variable-volume chamber in thedischarge configuration/phase of the apparatus/process.

In one aspect, the pumping energy of the liquid (typically water) of thecompensation circuit in discharge phase is between 1/100 and 7/100 ofthe discharge energy of the storage system through the turbine.

In one aspect, the secondary heat exchanger and the primary heatexchanger are configured to perform a sub-critical transformation of theworking fluid so that the working fluid is accumulated in the tank inliquid phase.

In one aspect, it is provided for removing heat from the working fluidin the primary heat exchanger until it is brought to a temperature belowthe critical temperature in a T-S diagram and to a point on the leftportion of the Andrews bell.

In one aspect, it is provided for removing heat from the working fluidin the secondary heat exchanger by passing it through the saturatedvapor zone until it reaches the liquid phase.

Further features and advantages will appear in greater detail in thedetailed description of preferred, but not exclusive, embodiments of aplant and process for energy storage according to the present invention.

DESCRIPTION OF DRAWINGS

This description will be set out below with reference to the attacheddrawings, which are provided for indicative and non-limiting purposes,in which:

FIG. 1 schematically shows an embodiment of an energy storage plantaccording to the present invention;

FIG. 2 shows a variant of the plant of FIG. 1 ;

FIG. 3 is a T-S diagram showing a process according to the presentinvention implemented in the plants of FIG. 1 or 2 ;

FIG. 4 shows a further embodiment of an energy storage plant accordingto the present invention;

FIG. 5 shows a variant of the plant of FIG. 4 ;

FIG. 6 is a T-S diagram showing a process according to the presentinvention implemented in the plants of FIG. 4 or 5 ;

FIG. 7 is a T-Q diagram showing a part of the process according to thepresent invention implemented in the plants of FIG. 4 or 5 ;

FIGS. 8, 9 and 10 show respective variants of a portion of the plant ofFIG. 2 ;

FIGS. 11 and 12 show respective variants of a different portion of theplants in FIGS. 1, 2, 4 and 5 ;

FIG. 13 shows a further embodiment of an energy storage plant accordingto the present invention.

DETAILED DESCRIPTION

With reference to the attached figures, with the reference number 1, aplant for the storage of energy (energy storage) according to thepresent invention has been indicated overall.

The plant 1, for example, operates with a working fluid other thanatmospheric air. For example, plant 1 operates with a working fluidchosen from the group including: carbon dioxide CO₂, sulphurhexafluoride SF₆, nitrous oxide N₂O. In the following description, theworking fluid used in combination with described plant 1 is carbondioxide CO₂.

Plant 1 is configured to perform a closed cyclic thermodynamictransformation (TTC), first in one direction into a chargeconfiguration/phase and then in the opposite direction into a dischargeconfiguration/phase, in which plant 1 stores heat and pressure in thecharge configuration and generates electrical energy in the dischargeconfiguration.

With reference to FIG. 1 , plant 1 comprises a turbine 2 and acompressor 3 mechanically connected to a shaft of a singlemotor-generator 4. The motor-generator 4, the compressor 3 and theturbine 2 are arranged on a same axis. A shaft of the turbine 2 iscoupled to one end of the shaft of the motor-generator 4 by means ofconnection devices, e.g. of the clutch type, which make it possible toconnect and disconnect, on command, the turbine 2 to and from themotor-generator 4. Similarly, a shaft of the compressor 3 is coupled toan opposite end of the shaft of the motor-generator 4 by means ofconnection devices, e.g. of the clutch type, which allow the compressor3 to be connected to and disconnected, on command, from themotor-generator 4. In other embodiments not shown here, the motor isfirmly connected to the compressor 3 and the generator is firmlyconnected to the turbine 2. In such a case, the motor is permanentlyconnected to compressor 3 and the generator is permanently connected toturbine 2.

Plant 1 comprises a casing 5 preferably defined by a pressure-balloonmade of flexible material, e.g. PVC coated polyester fabric. Thepressure-balloon is placed on the earth's surface and is externally incontact with atmospheric air. The pressure-balloon delimits inside avolume configured to contain the working fluid at atmospheric pressureor substantially atmospheric pressure, i.e. in equilibrium of pressurewith the atmosphere. The casing 5 may also be designed as a gasometer orany other gas storage system with low or no overpressure.

First pipelines 6 develop between the casing 5 and an inlet 3 a of thecompressor 3 and between the casing 5 and an outlet 2 b of the turbine 2to connect the internal volume of the casing 5 with said compressor 3and turbine 2. A valve or a valve system, not illustrated, may beoperationally placed on the first pipelines 6 to put in fluidcommunication alternately the casing 5 with the inlet 3 a of thecompressor 3 or the outlet 2 b of the turbine 2 with the casing 5.

The plant 1 comprises a primary heat exchanger 7 which can beselectively put in fluid communication with an outlet 3 b of compressor3 or with an inlet 2 a of turbine 2. For this purpose, second pipelines8 develop between the inlet 2 a of the turbine 2 and primary heatexchanger 7 and between the outlet 3 b of the compressor 3 and theprimary heat exchanger 7. A valve, or a valve system, not illustrated,is operationally located on the second pipelines 8 to connect theprimary heat exchanger 7 with the inlet 2 a of turbine 2 or the outlet 3b of compressor 3 with the primary heat exchanger 7. In a preferredembodiment, there is only one valve or valve system located on thesecond pipelines 8.

A tank 9 is in fluid communication with the primary heat exchanger 7 andis configured to store the working fluid in liquid or supercriticalphase.

The tank 9 is preferably made of metal with a spherical outer wall.

A secondary heat exchanger 10 is operationally active between theprimary heat exchanger 7 and the tank 9, or in said tank 9, and isconfigured to operate on the working fluid accumulated or inaccumulation phase in the tank 9. According to what is shown in theembodiment of FIG. 1 , the secondary heat exchanger 10 is integrated intank 9 in the sense that it has its own heat exchange portion 11 housedinside the tank 9 and configured to be touched by the working fluidcontained in said tank 9. Third pipes 12 develop between the primaryheat exchanger 7 and the tank 9 to put in fluid communication saidprimary heat exchanger 7 with said tank 9 and with said secondary heatexchanger 10.

In the schematic representation of FIG. 1 , the plant 1 may alsocomprise an additional heat exchanger 13 operationally placed betweenthe casing 5 and the compressor 2 and between the casing 5 and theturbine 2 and possibly a cooler 13 a positioned on a branch of the firstpipelines 6 connected to the outlet 2 b of turbine 2.

The plant 1 also comprises a control unit, not shown, operationallyconnected to the different elements of the same plant 1 andconfigured/programmed to manage its operation.

The plant 1 is configured to operate in a charge configuration or in adischarge configuration or to perform a process comprising a phase ofenergy charge and a phase of discharge and energy generation.

In the charge configuration, the plant 1 starts from a first state inwhich the working fluid (CO₂) in gaseous form is all contained in thecasing 5 at atmospheric pressure or substantially atmospheric pressureand at a temperature substantially equal to the ambient temperature(point A of the T-S diagram in FIG. 3 ). Casing 5, through the valvesystem, is connected to the inlet 3 a of the compressor 3 whilecommunication with the outlet 2 b of the turbine 2 is blocked. Inaddition, by means of the valve system, the primary heat exchanger 7 isin fluid communication with the outlet 3 b of the compressor 3 andcommunication with the inlet 2 a of the turbine 2 is blocked. Themotor-generator 4 is coupled to the compressor 3 only and is decoupledfrom the turbine 2 (which is at rest) and works as motor to drive thecompressor 3 such as to compress the working fluid coming from thecasing 5.

Before entering the compressor 3, the working fluid passes through theadditional heat exchanger 13 which acts as a heater to pre-heat theworking fluid (point B of the T-S diagram in FIG. 3 ). The working fluidis then compressed in the compressor 3 and heats up (point C of the T-Sdiagram in FIG. 3 ). The working fluid then flows through the primaryheat exchanger 7 which works as a cooler to remove heat from thecompressed working fluid, cool it (point D of the T-S diagram in FIG. 3) and store the thermal energy removed from the working fluid. At pointD the working fluid is at a temperature lower than the criticaltemperature of the working fluid and at a point on the left side of theAndrews bell or slightly outside the bell in conditions of slightoverheating. This compression may be adiabatic, inter-cooled orisothermal.

The working fluid enters the tank 9 where the secondary heat exchanger10, which in this configuration works as a cooler, removes further heatfrom the working fluid and accumulates further thermal energy. Theworking fluid passes through the saturated vapor zone until it reachesthe liquid phase (point E of the T-S diagram in FIG. 3 ). The tank 9therefore accumulates the working fluid in the liquid phase at atemperature lower than its own critical temperature Tc. In this secondstate, the working fluid (CO₂, Tc=31° C.) in liquid form, for example at20° C., is all contained in the tank 9. The secondary heat exchanger 10and the primary heat exchanger 9 are therefore configured to perform asub-critical transformation of the working fluid so that the workingfluid is accumulated in the tank 9 in liquid phase.

In the discharge configuration, the plant 1 starts from the second state(point F of the T-S diagram in FIG. 3 ). The casing 5, through the valvesystem, is put in communication with the outlet 2 b of turbine 2 whilecommunication with the inlet 3 a of the compressor 3 is blocked. Inaddition, by means of the valve system, the primary heat exchanger 7 isin fluid communication with the inlet 2 a of the turbine 2 and thecommunication with the outlet 3 b of the compressor 3 is blocked. Themotor-generator 4 is coupled to turbine 2 only and is decoupled fromcompressor 3 (which is at rest) and works as a generator driven inrotation by the turbine 2 driven by the expanding working fluid.

The secondary heat exchanger 10 works as a heater and transfers some ofthe heat previously accumulated in the charge configuration to theworking fluid in the tank 9. The working fluid passes through thesaturated steam zone until it reaches the steam phase (point G of theT-S diagram in FIG. 3 ). The working fluid passes through the primaryheat exchanger 7 which now works as a heater and releases additionalheat, previously accumulated in the charge configuration, to the workingfluid and heats it (point H of the T-S diagram in FIG. 3 ).

The heated working fluid enters the turbine 2, expands and cools (pointI of the T-S diagram in FIG. 3 ) and causes the rotation of the turbine2. The turbine 2, rotated by the heated working fluid, drives themotor-generator 4, which works as a generator and generates electricalenergy. The working fluid expansion in the turbine may be adiabatic,inter-heated or isothermal.

The working fluid coming from turbine 2 is cooled in the additional heatexchanger 13 (point J of diagram T-S in FIG. 3 ) and returns into thecasing 5 at atmospheric or substantially atmospheric pressure. Theadditional heat exchanger 13 in this phase stores additional thermalenergy in a respective additional thermal energy storage device, whichwill be used in the next charge phase to pre-heat the working fluid.

In the transformation illustrated in FIG. 3 , the secondary circuit 20is configured to remove heat from the working fluid, in the chargeconfiguration, or to transfer heat to the working fluid, in thedischarge configuration, at a temperature close to the ambienttemperature, for example, of about 20° C.

Both in the charge and in the discharge configuration/phase, since thesecondary heat exchanger 10 operates in conditions close to the ambienttemperature, due to the fact that the fluid has a critical temperatureclose to the ambient temperature, it is possible that the heat removalphase and/or the heat supply phase by the secondary heat exchangeris/are assisted by a phase of direct or indirect exchange with theatmosphere.

For example, a working fluid temperature (CO₂) accumulated in the tank 9is 24° C. and a working fluid pressure accumulated in the tank 9 is 65bar. The density of CO₂ at 25° C. and atmospheric pressure is about 1.8kg/m³. The density of CO₂ in the tank 9 is about 730 kg/m³. The ratiobetween the density of the working fluid when contained in the tank 9under the indicated conditions and the density of the same working fluidwhen contained in the casing 5 under atmospheric conditions is thereforeabout 400. It should be noted in this regard that if instead of CO₂ theatmospheric air stored at 65 bar and 24° C. in the tank 9 were used, itsdensity would be only 78 kg/m³ and the volume of the tank 9theoretically required would be about ten times greater.

For example, for a plant 1 according to the invention able to store 100MWh of energy, the volume of the pressure-balloon is about 400000 m³while the volume of the tank is about 1000 m³.

The variant of FIG. 2 shows a type of primary heat exchanger 7, i.e. afixed bed heat regenerator comprising a thermal mass 14 consisting, forexample, of metal balls. In the charge configuration/phase, the thermalmass 14 is lapped by the hot, compressed working fluid, which transfersheat to the metal balls that store thermal energy. In the dischargeconfiguration/phase, the thermal mass 14 is lapped by the cold workingfluid, which absorbs heat from the metal balls and heats up. In avariant not shown, the heat regenerator may also be of the moving bedtype. The primary heat exchanger 7 is therefore a thermal storage(Thermal Energy Storage TES).

Instead of the fixed bed heat regenerator shown in FIG. 2 , other typesof heat regenerator may be used.

For example, a possible primary heat exchanger 7 is shown in FIG. 11 .As shown in FIG. 11 , the primary heat exchanger 7 comprises a primarycircuit 15 crossed by a primary fluid, such as water, oil or salts. Theprimary circuit 15 comprises a heat exchange portion 16 configured toexchange heat with the working fluid. For example, in the schematicembodiment illustrated above, a section of the second 8 pipelinesthrough which the working fluid flows passes through the heat exchangesection 16, so that the primary fluid touches said section. The primarycircuit 15 comprises a hot primary storage chamber 17, for the hotprimary fluid accumulated after removing heat from the working fluid inthe charge configuration/phase of the apparatus/process, and a coldprimary storage chamber 18, for the cold primary fluid accumulated aftertransferring heat to the working fluid in the dischargeconfiguration/phase of the apparatus/process. The heat exchange portion16 is placed between the hot primary storage chamber 17 and the primarycold storage chamber 18. In the charge configuration/phase of theapparatus/process, the primary fluid flows from the cold primary storagechamber 18 to the hot primary storage chamber 17, removing heat from theworking fluid. In the discharge configuration/phase of theapparatus/process, the primary fluid flows from the hot primary storagechamber 17 to the cold primary storage chamber 18 releasing heat fromthe working fluid.

A different possible primary heat exchanger 7 is shown in FIG. 12 .According to what is illustrated in FIG. 12 , the primary circuit 15 ofthe primary heat exchanger 7 comprises a heat exchange portion 16defined by a section of primary circuit 15 that is lapped by the workingfluid that passes through the second pipelines 8. The primary circuit 15also comprises a fixed bed heat regenerator 19, preferably operating atatmospheric pressure and preferably similar to that described above,which is lapped by the primary fluid.

The variant of FIG. 2 is not equipped with the additional heat exchanger13 so that the corresponding T-S diagram, not illustrated, does notshow, with respect to the diagram of FIG. 3 , points B and J.

The variant in FIG. 2 also has a special structure of the secondary heatexchanger 10. The secondary heat exchanger 10 shown includes a secondarycircuit 20 crossed by a secondary fluid, such as air or water. Thesecondary circuit 20, in addition to the heat exchange portion 11 housedinside the tank 9, comprises a secondary hot storage chamber 21, for thesecondary hot fluid accumulated after removing heat from the workingfluid in the charge configuration/phase of the apparatus/process, and asecondary cold storage chamber 22, for the secondary cold fluidaccumulated after releasing heat to the working fluid in the dischargeconfiguration/phase of the apparatus/process. The above mentionedchambers 21, 22 are also connected to each other, in addition to theabove mentioned heat exchange portion 11, through a radiator 23 equippedwith fans 24 and with recirculation ducts that cools the secondary fluidduring the night and heats it during the day.

FIGS. 8, 9 and 10 show other variants of the secondary heat exchanger 10associated with tank 9.

In FIG. 8 , the secondary circuit 20, in addition to the heat exchangeportion 11, is equipped with an additional heat exchange portion 25through which it exchanges heat with e.g. air or sea water.

In FIG. 9 , the secondary circuit 20 is equipped with a secondary tank26 with water/ice or another two-phase system operationally connected toan auxiliary chiller 27.

In FIG. 10 , the secondary circuit 20 is located in a basin full ofwater consisting of several chambers 28 a, 28 b, 28 c. The embodimentillustrated in FIG. 10 shows a chamber 28 a for hot water storage, achamber 28 b for cold water storage and a chamber 28 c in fluidcommunication with the others and housing part of the secondary circuit20. The secondary fluid in the secondary circuit 20 is cooled or heatedby the water in the basin. The working fluid is condensed in the chargephase and evaporated in the discharge phase by the suitably circulatedwater, preferably through immersion pumps and through the secondaryfluid. The chambers 28 of said basin can be covered or uncovered and incommunication or not with the environment so that the chamber from whichthe water is circulated for condensation during charging is alwayscooled by the surrounding environment, by appropriate panels 29, whilethe one from which the water is circulated for evaporation duringdischarging is always heated by the surrounding environment and possiblykept warm by a cover. The above can be further supported by specialexchange systems that absorb heat or release heat, both throughconvection and radiation, with the environment, all in order to improvethe RTE of the system.

The embodiments of FIGS. 4 and 5 differ structurally from what hasalready been described because the secondary heat exchanger 10 is placedbetween the primary heat exchanger 7 and the tank 9, i.e. it is notintegrated in the tank 9. The secondary heat exchanger 10 is in line onthe third pipelines 12. FIG. 4 schematically illustrates a genericsecondary heat exchanger 10. FIG. 5 shows a schematic design example ofthe secondary heat exchanger 10.

The secondary heat exchanger 10 shown in FIG. 5 comprises a secondarycircuit crossed by a secondary fluid, e.g. water. The secondary circuit20 has a heat exchange portion 11 that is lapped by the working fluidthat passes through the third pipelines 12 and is configured to exchangeheat with the working fluid.

The secondary circuit 20 of FIG. 5 comprises a secondary hot storagechamber 21, for the secondary hot fluid accumulated after removing heatfrom the working fluid in the charge configuration/phase of theapparatus/process, and a secondary cold storage chamber 22, for thesecondary cold fluid accumulated after releasing heat to the workingfluid in the discharge configuration/phase of the apparatus/process.

The heat exchange portion 11 is located between the secondary hotstorage chamber 21 and the secondary cold storage chamber 22. In thecharge configuration/phase of the apparatus/process, the secondary fluidflows from the secondary cold storage chamber 22 to the secondary hotstorage chamber 21, removing heat from the working fluid. In thedischarge configuration/phase of the apparatus/process, the secondaryfluid flows from the secondary hot storage chamber 21 to the secondarycold storage chamber 21, releasing heat from the working fluid. Thesecondary circuit 20 also comprises one or more intermediate secondarystorage chambers 30 to adjust/vary the flow rate of the secondary fluidin the heat exchange portion 11 and the temperature variation of theworking fluid exchanging heat with this secondary fluid. FIG. 5 showstwo intermediate secondary storage chambers 30.

The embodiments of FIGS. 4 and 5 differ structurally from what hasalready been described also because the tank 9 comprises a separationmembrane 31 configured to internally separate the tank 9 in a firstchamber with variable volume 32 for the working fluid in super-criticalphase and in a second chamber with variable volume 33 in fluidcommunication with a compensation circuit 34 containing water. Thecompensation circuit 34 is configured to maintain a substantiallyconstant pressure in the super-critical working fluid coming from thesecondary heat exchanger 20 and contained in the first variable volumechamber 32 of the tank 9.

The compensation circuit 34 comprises an auxiliary tank 35 for water atatmospheric pressure, which is in fluid communication, throughappropriate pipelines, with a lower portion of the tank 9 and with thesecond variable volume chamber 33. An auxiliary turbine 36 has an inletin communication with the second variable volume chamber 33 and anoutlet connected to the auxiliary tank 35. The auxiliary turbine 36 isconnected to an auxiliary generator 37 and is configured to be rotatedby the water coming from the second variable volume chamber 33 in thecharge configuration/phase of the apparatus/process. A pump 38 has aninlet in communication with the auxiliary tank 35 and an outletconnected to the second variable volume chamber 33. The pump 38 isconnected to an auxiliary motor 39 and is configured to pump water fromthe auxiliary tank 35 into the second variable volume chamber 33 in thedischarge configuration/phase of the apparatus/process.

FIG. 6 shows the T-S diagram for the embodiments of FIGS. 4 and 5 .

FIG. 7 shows the T-Q diagram relating to a part of the thermodynamictransformation carried out by the embodiment of FIG. 5 .

The secondary heat exchanger 10 and the primary heat exchanger 7 of theembodiments of FIGS. 4 and 5 are configured to operate a super-criticaltransformation of the working fluid so that said working fluid isaccumulated in the tank in super-critical phase. In fact, unlike what isshown in FIG. 3 , the primary heat exchanger 7 removes heat from theworking fluid up to bring it (point D of FIG. 6 ) to a temperaturehigher than the critical temperature and above the Andrews bell.

Subsequently, the secondary heat exchanger 10 brings the working fluidinto super-critical phase (point E) making it follow the right side ofAndrews' bell. FIG. 7 shows the temperature decrease from point D topoint E of the working fluid during the charge phase and thecorresponding temperature increase of the secondary working fluid of thesecondary heat exchanger 10 of FIG. 5 (points U, V, W, Z).

The same FIG. 7 also illustrates the temperature increase from point Fto point G of the working fluid during the discharge phase and thecorresponding temperature decrease of the secondary working fluid of thesecondary heat exchanger 10 of FIG. 5 (points Z, W, V, U).

For instance, a working fluid temperature (CO₂) accumulated insuper-critical phase in the tank 9 is 25° C. and a working fluidpressure accumulated in super-critical phase in the tank 9 is 100 bar.The density of CO₂ at 25° C. and atmospheric pressure is about 1.8kg/m³. The density of CO₂ in the tank 9 is about 815 kg/m³. The ratiobetween the density of the working fluid when contained in the tank 9under the indicated conditions and the density of the same working fluidwhen contained in the casing 5 under atmospheric conditions is thereforeabout 450.

It should be noted that the structure of the secondary heat exchanger ofFIG. 10 may also be adopted in the embodiment of FIGS. 4 and 5 .

In addition, the secondary heat exchanger may be equipped with flow rateand/or temperature control systems for secondary fluid, typically wateror air, capable of regulating the pressure in the storage tanks withincertain limits, when the system operates in sub-critical conditions.Temperature control may, for example, be carried out by adding heat fromthe atmosphere or removing heat to atmosphere, also taking advantage ofthe normal fluctuations in the ambient temperature of air and water atdifferent times of the day.

In the illustrated embodiments using CO₂ as the working fluid, a CO₂dehydration system, a de-humidifier, for example with zeolites, is alsopreferably present to avoid potential formation of carbonic acid in thecircuit.

FIG. 13 shows a further variant of plant 1. It shows the main elementscommon to FIG. 1 , i.e. the turbine 2, the compressor 3, the motorgenerator 4, the casing 5, the primary heat exchanger 7 (TES thermalstorage), the tank 9 and the secondary heat exchanger 10. Plant 1 shownhere also comprises the additional heat exchanger 13. As in theembodiment shown in FIG. 4 , the secondary heat exchanger 10 is locatedbetween the primary heat exchanger 7 and the tank 9, i.e. it is notintegrated in tank 9. Similar to the plant shown in FIG. 2 , thesecondary heat exchanger 10 comprises a secondary circuit 20 crossed bya secondary fluid, e.g. water. The secondary circuit 20, in addition tothe heat exchange portion 11 comprises a secondary storage chamber 200,for the secondary hot fluid accumulated after removing heat from theworking fluid in the charge configuration/phase of the apparatus/processand for the secondary cold fluid accumulated after releasing heat to theworking fluid in the discharge configuration/phase of theapparatus/process. The above mentioned secondary storage chamber 200 isalso combined with a radiator 23 equipped with one or more fans 24placed on a recirculation duct which, for example, cools the secondaryfluid during the night and heats it during the day. The above-mentionedsecondary storage chamber 200 is also connected to the additional heatexchanger 13 via a corresponding circuit 210.

In this embodiment, plant 1 also comprises at least one additional heatexchanger 220 which receives heat from an additional heat source 230.The additional heat exchanger 220 is located on the second pipeline 8,between the inlet 2 a of turbine 2 and the primary heat exchanger 7. Theadditional heat source 230 is, for example but not exclusively, a solarsource (e.g. solar field), residual heat deriving from industrialrecovery (Waste Heat Recovery), exhaust heat from gas turbines, etc.

The additional heat source 230 provides additional heat during thedischarge phase.

The temperature at which the working fluid is brought during thedischarge phase and just before it enters turbine 2, via the additionalheat source 230 and the additional heat exchanger 220, is higher thanthe temperature of the working fluid that is obtained at the end of thecompression during the charge phase. For example, the temperature atwhich the working fluid is brought by the additional heat source 230 andthe additional heat exchanger 220 is about 100° C. but also 200° C. or300° C. or 400° C. higher than the temperature of the working fluid atthe end of compression.

Plant 1 is also equipped with an auxiliary thermal storage 240 (ThermalEnergy Storage TES) connected, through appropriate circuits, to thecompressor 2 and the turbine 2 in order to achieve, in the compressor 3(during the charge phase), an inter-cooled compression (with one or moreinter-coolings) and to achieve, in the turbine 2 (during the dischargephase), an inter-heated expansion (with one or more inter-heatings). Theheat accumulated in the auxiliary heat accumulator 240 during theinter-cooled compression is used in whole or in part to achieve theinter-heated expansion.

In an embodiment of the process performed with the plant of FIG. 13 , itis provided not to carry out inter-coolings in the charge phase and notto carry out inter-heatings in the discharge phase and to provideadditional heat in the discharge phase through the additional heatsource 230 and the additional heat exchanger 220.

In variants of the process performed with the plant of FIG. 13 , it isprovided to make one or more inter-coolings in the charge phase and anequal number of inter-heatings in the discharge phase, in addition toproviding additional heat in the discharge phase through the additionalheat source 230 and the additional heat exchanger 220.

In a further embodiment of the process performed with the plant of FIG.13 , it is provided to carry out a number of inter-coolings in thecharge phase and to carry out a single inter-cooling in the dischargephase using the heat (accumulated in the auxiliary thermal accumulator240) only of the last inter-cooling, in addition to heat with theadditional heat through the additional heat source 230 and theadditional heat exchanger 220. The heat stored in the auxiliary heatstorage 240 and coming from the remaining intercoolings can be used forother purposes, e.g. for co-generation.

LIST OF ELEMENTS

-   1 energy storage plant-   2 turbine-   2 a turbine inlet-   2 b turbine outlet-   3 compressor-   3 a compressor inlet-   3 b compressor outlet-   4 motor-generator-   5 casing-   6 first pipelines-   7 primary heat exchanger-   8 second pipelines-   9 tank-   10 secondary heat exchanger-   11 heat exchange portion of secondary heat exchanger-   12 third pipelines-   13 additional heat exchanger-   13 a cooler-   14 thermal mass-   15 primary circuit-   16 heat exchange portion of the primary circuit-   17 primary hot storage chamber-   18 primary cold storage chamber-   19 fixed bed heat regenerator-   20 secondary circuit-   21 secondary hot storage chamber-   22 secondary cold storage chamber-   23 radiator-   24 fans-   25 further heat exchange portion-   26 secondary tank-   27 auxiliary chiller-   28 a, 28 b, 28 c water basin chambers-   29 panels-   30 intermediate secondary storage chambers-   31 separation membrane-   32 first variable volume chamber-   33 second variable volume chamber-   34 compensation circuit-   35 auxiliary tank-   36 auxiliary turbine-   37 auxiliary generator-   38 pump-   39 auxiliary motor-   200 secondary storage chamber-   210 additional heat exchanger circuit-   220 additional heat exchanger-   230 additional heat source-   240 auxiliary thermal storage

The invention claimed is:
 1. An energy storage plant, comprising: acasing configured to store a working fluid other than atmospheric air ina gaseous phase; a tank configured to store said working fluid in aliquid phase; a compressor; a turbine; and heat exchangers configured tostore thermal energy from the working fluid or to release thermal energyto the working fluid, wherein the casing is selectively in fluidcommunication with an inlet of the compressor or with an outlet of theturbine; wherein the heat exchangers are selectively in fluidcommunication with an outlet of the compressor or with an inlet of theturbine; wherein the tank is in fluid communication with the heatexchangers; wherein the plant is configured to perform a closed cyclicthermodynamic transformation, first in one direction in a chargeconfiguration and then in an opposite direction in a dischargeconfiguration, between said casing and said tank; wherein in the chargeconfiguration the plant stores heat and pressure and in the dischargeconfiguration generates energy; wherein the heat exchangers areconfigured to operate a sub-critical transformation of the workingfluid; wherein the plant is configured to make the tank work at aconstant pressure or substantial constant pressure and to make thecasing work at a constant pressure or substantial constant pressure;wherein in the charge configuration the working fluid is accumulated inthe tank in the liquid phase with a temperature close to a criticaltemperature and said critical temperature is close to the ambienttemperature; wherein in the discharge configuration the working fluid isevaporated, expanded, and stored in the casing; wherein the casing is apressure-balloon or has the structure of a gasometer so that the workingfluid in said casing is in equilibrium of pressure with the atmosphere,with low or no overpressure, in any operating condition, wherein theheat exchangers comprise a primary heat exchanger and a secondary heatexchanger, the secondary heat exchanger being located between theprimary heat exchanger and the tank or being integrated into the tank;and wherein, in the discharge configuration, the working fluid is heatedthrough the heat stored by the primary heat exchanger and secondary heatexchanger.
 2. The plant of claim 1, wherein the secondary heat exchangeris configured to regulate the pressure in the tank in the chargeconfiguration and in the discharge configuration through a direct orindirect heat exchange with the atmosphere.
 3. The plant of claim 1,wherein the primary heat exchanger is a thermal energy storage or isoperatively coupled to a thermal energy storage; and wherein the thermalenergy storage is configured to store thermal energy released by theworking fluid in the charge configuration and to provide thermal energyto the working fluid in the discharge configuration.
 4. The plant ofclaim 1, wherein the secondary heat exchanger comprises a secondarycircuit crossed by a secondary fluid; wherein the secondary circuitincludes a heat exchange portion lapped by the working fluid, asecondary hot storage chamber, for the secondary fluid accumulated afterremoving heat from the working fluid in the charge configuration, and asecondary cold storage chamber, for the secondary fluid accumulatedafter releasing heat to the working fluid in the dischargeconfiguration; and wherein the secondary hot storage chamber and thesecondary cold storage chamber are connected to each other through aradiator equipped with fans and with recirculation ducts that cool thesecondary fluid during night and heats it during day.
 5. The plant ofclaim 1, wherein the secondary heat exchanger comprises a secondarycircuit crossed by a secondary fluid; wherein the secondary circuitincludes a heat exchange portion lapped by the working fluid and anadditional heat exchange portion; and wherein the additional heatexchange portion is configured to exchange heat with air or sea water.6. The plant of claim 1, wherein the secondary heat exchanger comprisesa secondary circuit crossed by a secondary fluid; and wherein thesecondary circuit includes a heat exchange portion lapped by the workingfluid and a secondary tank with a two-phase system operationallyconnected to an auxiliary chiller.
 7. The plant of claim 1, wherein thesecondary heat exchanger comprises a secondary circuit crossed by asecondary fluid; wherein the secondary circuit includes a heat exchangeportion lapped by the working fluid; wherein the secondary circuit islocated in a basin full of water consisting of several chambers; andwherein the secondary fluid in the secondary circuit is cooled or heatedby the water in the basin.
 8. The plant of claim 1, wherein thesecondary heat exchanger comprises a secondary circuit crossed by asecondary fluid; wherein the secondary circuit includes a heat exchangeportion lapped by the working fluid and a secondary storage chamber, forthe secondary fluid accumulated after removing heat from the workingfluid in the charge configuration and for the secondary fluidaccumulated after releasing heat to the working fluid in the dischargeconfiguration; and wherein the secondary storage chamber is combinedwith a radiator equipped with one or more fans placed on a recirculationduct which cools the secondary fluid during night and heats it duringday.